Many studies have been reported on a so-called sleep-apnea syndrome, i.e. apnea which is accompanied by the suspension of breathing, for example, for 10 seconds or more in one's sleep at night.
If breathing comes to stop a few dozen times to hundreds of times during sleep, lack of oxygen in the body of a patient reaches a serious level, thus displaying symptoms such as “insomnia” and “choking” during sleep. As a result, the patient falls into a state of lack of sleep.
Accordingly, the patient gets sleepy in the daytime, leading to low concentration and a low energy level, or to dozing while working, which can cause serious accidents, such as those caused by drowsy driving.
Moreover, lack of oxygen will place undue stress on circulation organs, leading to the increased incidence of abnormal cardiac rhythm, high blood pressure, heart failure and diabetes. Thus, the respiratory abnormality in one's sleep is a clinically important subject, and it is necessary to take measures for preventing the occurrence of the above-mentioned disorders.
The sleep-apnea syndrome is classified into a so-called central type caused by an abnormality in the respiratory center, an obstructive type by an upper airway obstruction, and a mixed type by a combination thereof.
As for the obstructive type that often causes the sleep-apnea syndrome, there have been conventionally known methods of treatment for opening the closed upper airway, by, for example, putting a mouthpiece into a patient's mouth to fix a lower jaw in such a manner protruding forward, or by letting a patient put on a plastic nasal cavity mask at the time of sleeping, and then pumping air from a pumping installation connected with the nasal cavity mask through a hose.
According to the former method, however, since the patient cannot take breaths through the mouth with the mouthpiece put therein, it can not be used when he/she has nasal congestion. According to the latter method, the patient must put on a nasal cavity mask for feeding air to his/her face, and thus there is a possibility that the patient may experience discomfort during sleep.
To address the foregoing problems, Japanese Registered Patent Publication No. 2794196 proposes an apnea preventive stimulating device in which a respiratory condition of a patient is detected by a respiration detection device such as a thermistor, and if the respiration detection device determines that the patient is in a respiratory standstill, then stimulating signals comprising electric pulses of a frequency of 40 to 150 Hz, a peak value of 1 to 50 volts and rise-up time constant of 0.2 seconds or more, are applied to his/her genioglossus muscle, which is one of the dilator muscles of the closed upper airway.
The apnea preventive stimulating device of the foregoing structure is advantageous in that since it applies stimulating signals to genioglossus muscle without using an air pressure, it is not necessary to put on a nasal cavity mask covering a substantial area on a face, and that the upper airway can be recovered from obstruction promptly, irrespective of whether the patient has nasal congestion or not.
According to the device, however, the stimulation signals are only applied when the patient is determined to be in the respiratory standstill after detecting his/her respiratory condition, and thus two or more thermistors need to be attached to the neighborhood of both nostrils or a mouth of the patient. Besides, the patient is liable to be wakened due to the stimulation signals being applied synchronously with the respiratory standstill of the patient.